Enhancement of electron and hole mobilities in &lt;110&gt; Si under biaxial compressive strain

ABSTRACT

The present invention provides a semiconductor material that has enhanced electron and hole mobilities that comprises a Si-containing layer having a &lt;110&gt; crystal orientation and a biaxial compressive strain. The term “biaxial compressive stress” is used herein to describe the net stress caused by longitudinal compressive stress and lateral stress that is induced upon the Si-containing layer during the manufacturing of the semiconductor material. Other aspect of the present invention relates to a method of forming the semiconductor material of the present invention. The method of the present invention includes the steps of providing a silicon-containing &lt;110&gt; layer; and creating a biaxial strain in the silicon-containing &lt;110&gt; layer.

RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.60/534,916, filed Jan. 7, 2004, the entire content of which isincorporated herein by reference.

DESCRIPTION

1. Field of the Invention

The present invention relates to semiconductor materials having enhancedelectron and hole mobilities, and more particularly, to semiconductormaterials that include a silicon (Si)-containing layer having enhancedelectron and hole mobilities. The present invention also providesvarious methods of forming such semiconductor materials.

2. Background of the Invention

For more than three decades, the continued miniaturization of siliconmetal oxide semiconductor field effect transistors (MOSFETs) has driventhe worldwide semiconductor industry. Various showstoppers to continuedscaling have been predicted for three decades, but a history ofinnovation has sustained Moore's Law in spite of many challenges.However, there are growing signs today that metal oxide semiconductor(MOS) transistors are beginning to reach their traditional scalinglimits [A concise summary of near-term and long-term challenges tocontinued CMOS scaling can be found in the “Grand Challenges” section ofthe 2002 Update of the International Technology Roadmap forSemiconductors (ITRS). A very thorough review of the device, material,circuit, and systems limits can be found in Proc. IEEE, Vol. 89, No. 3,March 2001, a special issue dedicated to the limits of semiconductortechnology].

Since it has become increasingly difficult to improve MOSFET andtherefore complementary metal oxide semiconductor (CMOS) circuitperformance through continued miniaturization, methods for improvingperformance without scaling have become critical. One general approachfor doing this is to increase carrier (electron and/or hole) mobilities.This can by done by either: (1) introducing an appropriate strain intothe Si lattice; (2) by building MOSFETs on Si surfaces that are orientedin directions different than the conventional <110> Si; or (3) acombination of (1) and (2).

As far as approach (1) is concerned, several methods such as, forexample, strained Si on a relaxed SiGe buffer layer and strained Si onrelaxed SiGe on insulator have been described for producing Si underbiaxial tensile strain. This has been shown to significantly enhanceelectron mobilities, but requires high Ge fractions to only mildlyenhance hole mobilities in <100> Si.

In terms of approach (2), it is well known that hole mobilities in <110>Si are more than twice that of conventional <100> Si. However, electronmobilities in relaxed (unstrained) <110> Si are degraded by about afactor of two compared to the <100> case. This has led to the inventionof a somewhat complex “hybrid” scheme for integrating pFETs built in<110> Si and nFETs built in <100> Si [M. Yang et al., IEDM TechnicalDigest, pg. 453, 2003]. Although this hybrid approach benefits pFETssignificantly, it typically has no benefit for nFETs.

There is a significant advantage to an approach that can significantlyenhance both electron and hole mobilities, while at the same timeavoiding the complexities of hybrid crystalline orientation schemes.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor material that hasenhanced carrier mobilities that comprises a Si-containing layer havinga <110> crystal orientation that is under a biaxial compressive strain.The term “biaxial compressive strain” is used herein to describe the netstress caused by longitudinal compressive stress and lateral (ortransverse) compressive stress that is induced in the plane of theSi-containing layer during the manufacturing of the semiconductormaterial.

The semiconductor material of the present invention that includes a<110> Si-containing layer with biaxial compressive strain providesenhanced mobilities for both nMOS and pMOS.

Another aspect of the present invention relates to a method of formingthe semiconductor material of the present invention. Specifically and inbroad terms, the method of the present invention includes the steps ofproviding a silicon-containing <110> layer; and creating a biaxialcompressive strain in the silicon-containing <110> layer.

In one embodiment, the Si-containing layer having the <110> orientationand biaxial compressive strain is created by a method that includes thesteps of:

forming at least one porous Si layer having an uppermost surface in a<110> Si-containing substrate;

annealing the uppermost surface so as to create a non-porous surfacelayer;

forming a crystalline epitaxial Si-containing layer having a <110>orientation on the non-porous surface layer thereby forming a transferstructure;

bonding the transfer structure to a material that has a highercoefficient of thermal expansion than Si at a temperature that iselevated above the ultimate device operating temperature to provide abonded structure;

cooling the bonded structure so that a mechanically weak interface formsat said at least one porous Si layer thereby cleaving said bondedstructure at said interface; and

removing remaining portions of the least one porous Si layer from thecleaved structure to provide a semiconductor material that includes atleast the crystalline epitaxial Si-containing layer having a <110>orientation atop said material, said crystalline epitaxial Si-containinglayer is under biaxial compressive strain.

In another embodiment, the Si-containing layer having the <110>orientation and biaxial compressive strain is created by a method thatincludes the steps of:

forming at least one multiply connected trench isolation region in asurface of a Si-containing layer having a <110> crystal orientation; and

forming at least one CMOS device on exposed portions of theSi-containing layer surrounded by said at least one multiply connectedtrench isolation region, wherein said at least one multiply connectedtrench isolation region creates biaxial compressive strain in saidSi-containing layer.

In yet another embodiment of the present invention, the Si-containinglayer having the <110> orientation and biaxial compressive strain iscreated by a method that includes the steps of:

providing a structure comprising a Si-containing layer having a <110>crystal orientation, said Si-containing layer having at least one CMOSdevice thereon; and

forming a compressive liner on said structure, wherein said compressiveliner causes said Si-containing layer to be under a biaxial compressivestrain under the gate of the CMOS device.

In yet another embodiment of the present invention, the Si-containinglayer having the <110> orientation and biaxial compressive strain iscreated by a method that includes the steps of:

forming at least one multiply connected trench isolation region in asurface of a Si-containing layer having a <110> crystal orientation;

forming at least one CMOS device on exposed portions of theSi-containing layer surrounded by said at least one multiply connectedtrench isolation region; and

forming a compressive liner on said Si-containing layer, wherein saidcompressive liner and said least one multiply connected trench isolationregion cause said Si-containing layer to be under a biaxial compressivestrain.

For the at least one multiply connected trench isolation region and thecompressive liner, the stress is primarily uniaxial when the devices arewide. As the width of the devices is reduced, the stress becomesbiaxial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A–1B are graphs of the electron mobility (cm²/Vs) vs. electronconcentration.

(cm⁻²) for a <100> Si substrate having a conventional orientation andcurrent flow direction (FIG. 1A), and for a Si substrate material havinga <110> orientation with a 1% biaxial compressive strain (FIG. 1B);other strains are also shown.

FIGS. 2A–2B are graphs of the hole mobility (cm²/Vs) vs. holeconcentration (cm⁻²) for a <100> Si substrate having a conventionalorientation and current flow direction (FIG. 2A), and for a Si substratematerial having a <110> orientation with a 1% biaxial compressive strain(FIG. 2B); other strains are also shown.

FIGS. 3A–3E are pictorial representations (through cross sectionalviews) illustrating the basic processing steps employed in a firstembodiment of the present invention.

FIGS. 4A–4C are pictorial representations (through cross sectionalviews) illustrating the basic processing steps employed in an embodimentof the present invention in which both an at least one multiplyconnected trench isolation region and a compressive liner are employedto create strain in a Si-containing layer, note that the <110> directionis perpendicular to the Si-containing substrate shown.

FIG. 5 shows the strain effect on CMOS performance.

FIGS. 6A–6B are graphs illustrating the STI mechanical stress effect ondrive current with different crystal orientation and different nitrideliner stress. All the devices have narrow widths (120 nm) and nominal(45 nm) length; FIG. 6A is for nMOS devices, and FIG. 6B is for pMOSdevices.

FIGS. 7A–7B are graphs illustrating the STI mechanical stress effect ofdevices with different widths, different crystal orientation anddifferent nitride liner stress; FIG. 7A is for nMOS devices, and FIG. 7Bis for pMOS devices.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a semiconductor materialcomprising a Si-containing layer having a <110> crystal orientation andbiaxial compressive strain and various methods of forming the same, willnow be described in greater detail by referring to the drawings thataccompany the present application.

The applicants of the present application have determined throughnumerical calculations that when a significant (greater than about 0.2%,preferably greater than about 0.5%) biaxial compressive strain isintroduced in a <110> Si-containing layer, both the electron and holemobilities exceed those in the conventional unstrained <100> Si case.The % strain is defined herein as the percentage change in thecrystalline lattice constant of a material in a given direction. Thesignificant advantages of combining both biaxial compressive strain witha <110> Si-containing layer has not previously been recognized in theart.

The results of the above calculations, which have been determined bycomputing the carrier mobilities using the Kubo-Greenwood formula (whichresults from a solution of the linearized Boltzmann transport equation)for carriers in inversion layers is shown in FIGS. 1A–1B and FIGS.2A–2B. The sub-band structure has been calculated using a model bandstructure consisting of six ellipsoidal conduction-band valleys (withfirst-order nonparabolic corrections following Kane) for nFETs, or bysolving the six-band k*p Hamiltonian (3 bands times 2 spin states) withspin-orbit interaction for the pFETs. In the case of nFETs, the effectof strain has been accounted for by allowing for the knowndegeneracy-breaking and energetic shifts of the conduction ellipsoidsand the (linear) changes of the effective masses. A full strainHamiltonian has been added to the total (k*p+spin-orbit) Hamiltonian inthe case of nFETs. The carrier momentum relaxation rates due to(intraband, intra- and inter-subband) scattering with acoustic phonons(in the elastic, equipartition approximation, valid for temperatureslarger than about 150K) and to inelastic, anisotropic scattering withoptical phonons (intervalley for nFETs, intra- and interband for pFETS)have been evaluated numerically using Fermi Golden Rule and deformationpotentials obtained from previous bulk calculations (M V Fischetti and SE Laux, J. Appl. Phys. 80, 2234 (1996)). Scattering with roughness atthe Si—SiO₂ interface has been treated according to the full Ando'smodel and using a rigorous multi-subband model for dielectric screening.

FIG. 1A shows the calculated electron mobility in the inversion layer ofnFETs (as a function of carrier sheet density) for a more common Si<100> wafer surface along the [110] crystallographic direction, usuallyemployed in present VLSI technology. The application of 1% biaxialtensile strain shows the well-known enhancement of the electron mobilityat low electron densities. By contrast, FIG. 1B shows that even moderateamounts of compressive strain (0.5% or larger) for <110> surfaces booststhe electron mobility (along the [110] direction) above and beyond thevalues attained at all densities for the relaxed or strained <100>surface.

As shown by comparing FIG. 1A and FIG. 1B, the application of 1%compressive strain on <110> Si surfaces enhances the electron mobilityby a factor of approximately 2 above the mobility obtained for <100>relaxed (or with 1% compressive or tensile strain) Si.

FIGS. 2A–2B present analogous information regarding the calculated holemobility for <100> (FIG. 2A) and <110> (FIG. 2B) Si surfaces. As can beseen in these drawings, the application of 1% compressive strain on<110> surfaces boosts the hole mobility along the [110] direction by afactor of approximately 3 over the hole mobility for the relaxed <100>Si surface.

These enhanced mobilities obtained using the inventive Si substratesimultaneously enable higher performance nFETs and pFETs, and avoid thecomplexities of a hybrid crystalline orientation approach. The followingdescription, with reference to FIGS. 3A–3E illustrate one method thatcan be employed in the present invention through which a biaxialcompressive strain (greater than about 0.2%, preferably greater thanabout 0.5%) can be introduced into a <110> Si-containing layer in orderto achieve these significantly higher carrier mobilities.

FIG. 3A illustrates an initial structure that can be used in forming theinventive substrate material of the present invention. Specifically, theinitial structure shown in FIG. 3A includes a <110> Si-containingsubstrate 10 having at least one porous Si layer 12 formed on thesurface of Si-containing substrate 10. The at least one porous Si layer12 has an uppermost surface layer 13. In the drawings, two porous Silayers 12A and 12B are formed. Despite showing the presence of twoporous Si layers 12A and 12B, the present invention works equally wellwhen only one porous Si layer or more than two porous Si layers areformed.

The term “Si-containing substrate” is used in the present invention todenote a semiconductor material that includes Si. Illustrative examplesof such Si-containing materials that can be employed as substrate 10include bulk Si, SiGe having a Ge content of about 25% or less,silicon-on-insulators (SOIs) and SiGe-on-insulators. The substrates canbe doped or undoped.

The at least one porous Si layer is formed in the present invention byutilizing an electrolytic anodization process that is capable ofconverting a surface portion of the <110> Si-containing substrate 10into a porous Si layer. The anodization process is performed byimmersing the <110> Si-containing substrate 10 into an HF-containingsolution while an electrical bias is applied to the <110> Si-containingsubstrate 10 with respect to an electrode also placed in theHF-containing solution. In such a process, the <110> Si-containingsubstrate 10 itself typically serves as the positive electrode of theelectrochemical cell, while another semiconducting material such as Si,or a metal is employed as the negative electrode.

The anodization process used in forming the porous Si layers can also bereferred to as an anodic etching process. The porous Si layers createdusing the anodization process are mechanically weak as compared to theremainder of the Si-containing substrate 10, yet the porous Si layerspreserve the crystalline quality and orientation of the Si-containingsubstrate 10.

It should be noted that when more than one porous Si layer 12 is formed,the other porous layers can have the same or different pore morphology.Porous Si layers containing different pore morphologies can be formed inthe present invention by changing the current flow conditions during theanodization process.

In general, the HF anodization converts a surface region of theSi-containing substrate 10 into porous Si. The rate of formation and thenature of the porous Si so-formed (porosity and microstructure) aredetermined by both the material properties as well as the reactionconditions of the anodization process itself (current density, bias,illumination and additives in the HF-containing solution). Generally,the porous Si layers 12A and 12B formed in the present invention have aporosity of about 0.1% or higher.

The thickness of each porous Si layer 12 may vary depending on theanodization conditions employed. Typically, the thickness of each porousSi layer 12 formed in the present invention is from about 100 nm toabout several microns, with a thickness from about 300 to about 500 nmbeing more typical. Each porous Si layer 12 may have the same ordifferent thickness that is within the ranges mentioned above.

The term “HF-containing solution” includes concentrated HF (49%), amixture of HF and water, a mixture of HF and a monohydric alcohol suchas methanol, ethanol, propanol, etc, or HF mixed with at least oneconventional surfactant. The amount of surfactant that is present in theHF solution is typically from about 1 to about 50%, based on 49% HF.

The anodization process is performed using a constant current sourcethat operates at a current density from about 0.05 to about 50milliAmps/cm². A light source may be optionally used to illuminate thesample. More preferably, the anodization process of the presentinvention is employed using a constant current source operating at acurrent density from about 0.1 to about 5 milliAmps/cm².

The anodization process is typically performed at room temperature or ata temperature that is slightly elevated from room temperature may beused. Following the anodization process, the structure is typicallyrinsed with deionized water and dried.

Following the anodization process in which at least one porous Si layer12 is formed in the <110> Si-containing substrate 10, the structureshown in FIG. 3A is subjected to an annealing process that is performedunder conditions (temperature and ambient) that are effective in sealingthe pores at the uppermost porous Si layer. In the present case shown,the annealing step would seal the pores at the surface of porous Silayer 12B. The annealing step performed at this point of the presentinvention causes surface diffusion of silicon atoms thereby creating athin skin of non-porous Si. The thin skin of non-porous Si is designatedby reference numeral 14 in FIG. 3B. The skin layer of non-porous Siformed at this point of the present invention generally has a thicknessfrom about 5 to about 80 nm, with a thickness from about 10 to about 30nm being more typical.

The annealing step that is used to seal the pores of the uppermostporous Si layer 13 is performed at high annealing temperatures. By “highannealing temperatures” it is meant annealing temperatures from about900° to about 1150° C. More preferably, the annealing step is performedat a temperature from about 1000° to about 1100° C. The annealing may beperformed using a single ramp up rate. Alternatively, the annealing maybe performed using varies ramp-up rates in which optional soak cyclescan be employed.

In addition to being performed at high temperatures, the annealing stepof the present invention, which is used to seal the pores of theuppermost porous Si layer 13, is also performed in the presence of ahydrogen-containing ambient. Suitable hydrogen-containing ambient thatcan be employed includes molecular or atomic hydrogen. In someembodiments, the hydrogen-containing ambient may be admixed with aninert gas such as He, Ar, N₂ or Xe. In some preferred embodiments of thepresent invention, the annealing ambient is H₂.

After sealing the pores at the top of the porous Si layer using theaforementioned high temperature annealing step, an epitaxial layer of aSi-containing material, i.e., Si or SiGe, is formed on the thin skinnedSi layer 14. The epitaxial Si-containing layer is a crystalline materialthat has the same crystal orientation as that of the substrate 10. Theepitaxial Si-containing layer is formed by employing an epitaxial growthprocess that is well known to those skilled in the art. For example, theepitaxial Si-containing layer can be formed by an ultra-high vacuumchemical vapor deposition (UHVCVD) process or any other like technique.

The epitaxial Si-containing layer formed atop the thin skinnednon-porous Si surface 14 is designed in FIG. 3B by reference numeral 16.The thickness of the epitaxial Si-containing layer 16 formed at thispoint of the present invention may vary depending on the process used informing the same. Typically, the epitaxial Si-containing layer 16 has athickness from about 10 to about 100 nm, with a thickness from about 10to about 30 nm being more typical.

It is noted that the above discussion regarding. Si formation, poresealing and epi growth is well known to one skilled in the art. Theabove processing steps are based on the well-known ELTRAN process forSOI wafer manufacture (see T. Yonehara and K. Sakaguchi, “ELTRAN (SOIEpi Water) Technology,” in The Science of SOI, Chapter 2, Section 2,(Apr. 19, 2000).

In some embodiments of the present invention, an optional oxide layer 18(see FIG. 3B) can be formed on the epitaxial Si-containing layer 16. Theoptional oxide layer 18 can be formed by a conventional oxidationprocess. Alternatively, the optional oxide layer 18 can be formed by aconventional deposition process such as chemical vapor deposition (CVD),plasma-enhanced chemical vapor deposition (PECVD), atomic layerdeposition (ALD), chemical solution deposition, and the like.

The thickness of the optional oxide layer 18 formed at this point of thepresent invention may vary depending on the process used in forming thesame. Typically, the optional oxide layer 18 has a thickness from about10 to about 200 nm, with a thickness from about 20 to about 100 nm beingmore typical.

The structure shown in FIG. 3B, with or without the optional oxide layer18, is employed in the present invention as a transfer structure thatwill be bonded to a substrate 20 at elevated temperature. FIG. 3C showssubstrate 20 which can be bonded to the optional oxide layer 18 or theepitaxial Si-containing layer 16 of the transfer structure. This bondingis achieved by first positioning the two structures as shown in FIG. 3C,bringing them in contact with each other, optionally applying anexternal force to the contacted structures, and heating the twostructures.

The substrate 20 that can be employed in the present invention includesany material that has a coefficient of thermal expansion that issignificantly greater than the Si-containing substrate 10. That is,substrate 20 includes any material having a coefficient of thermalexpansion a that is significantly greater than about 2.8 ppm/° C.Illustrative examples of suitable materials for substrate 20 includesapphire (α=8.8 ppm/° C.), germanium (α=5.8 ppm/° C. at roomtemperature, which increased significantly with temperature) and calciumfluoride (α=19 ppm/° C.).

In some embodiments not shown, an optional oxide layer can be formed onthe surface of the substrate 20 prior to bonding. This optional oxidelayer can be formed as described above and it alone or together with theoptional oxide layer 18 of the transfer structure can be used tofacilitate wafer bonding.

The heating step used to bond the two structures together is performedat an elevated temperature that ranges from about 400° to about 1000° C.More preferably, the bonding is performed at a temperature from about750° to about 925° C. The heating step can be performed using a singleramp-up rate or various ramp-up rates with optional soaking cycles canbe employed. In some embodiments, the heating step used to bond the twostructures together can be performed in an inert ambient including, forexample, He, Ar, N₂, Xe and mixtures therefore. Other ambients can bealso be used in bonding the two structures together.

Upon cooling down from the high wafer bonding temperature, substrate 20will contract more than Si-containing substrate 10 due to its highercoefficient of thermal expansion. This will create significantcompressive stress in the Si-containing layer 16 above the optionaloxide layer 18 (which will remain rigid at these temperatures) and inthe porous Si layers. The cooling down is typically performed using acool down rate of about 50° C./min or less.

Due to the considerable interfacial stress at the boundary between theporous layers, the bonded wafer will preferentially cleave along theinterface of the two porous layers. In FIG. 3D, reference numeral 22denotes the interface in which cleavage occurs. With one porous layer,the cleavage will occur within the porous layer or at the edge or theporous layer. Without the porous Si layer, either substrate 20 or theSi-containing substrate 10 will fracture due to the strong bonding andthe mismatch in coefficients of thermal expansion. This cleavage iswell-known to one skilled in the art of ELTRAN wafer fabrication.

Due to its extremely high surface-to-volume ratio, the remaining porousSi layer(s) can be removed with high selectivity (greater than 1000:1)to the epitaxial Si-containing layer 16 utilizing a wet etching process.In particular, the remaining porous Si layer not cleaved during the cooldown process can be removed using a wet etch process in which thechemical etchant is a solution of hydrofluoric acid, nitric acid andacetic acid. Other chemical etchants that can be employed in selectivelyremoving the remaining porous layers include a mixture of HF, H₂O₂, andH₂O. The selective removing of the remaining porous Si layers exposes asurface of epitaxial Si-containing layer 16.

FIG. 3E shows the structure that is formed after cleaving and removingof the remaining porous Si layers. The structure shown in FIG. 3Eincludes substrate 20, optional oxide layer 18 and epitaxialSi-containing layer 16 having a <110> orientation that is under biaxialcompressive strain. It is noted that the structure shown in FIG. 3E isan SOI like structure since the epitaxial Si-containing <110> layer 16is located directly upon an insulator, e.g., oxide layer 18.

The newly exposed Si-containing surface of layer 16 can be smoothed atthis point of the present invention utilizing an annealing process thatis carried out in an H₂-containing ambient. This annealing step isperformed at a temperature from about 850° to about 1100° C., with atemperature from about 900° to about 950° being more preferred. Careshould be taken during this annealing step so as not to relax thecompressively strained Si-containing layer 16 by flowing the oxide 18with an excessive (>1100° C.) thermal treatment. Chemical mechanicalpolishing (CMP) can also be used.

The thin Si-containing layer 16 is analogous to that formed in strainedSi directly on insulator (SSDOI) but with a strain of the opposite sign.The device scaling advantages that can be derived from the thin natureof Si-containing layer 16 would be similar to that of SSDOI, but withthe potential for even higher carrier mobility enhancements due to thesign of the strained and the orientation of the wafer.

In embodiments in which the optional oxide layer 18 is not present, thethin Si-containing layer 16 would be formed directly upon substrate 20.Note that since layer 16 is epitaxially grown it has the samecrystallographic orientation as substrate 10 which is <110>. In theembodiment in which substrate 20 is sapphire, the method of the presentinvention can lead to a biaxial compressive strain up to 0.6%. Inembodiments in which substrate 20 is calcium fluoride, the method of thepresent invention can lead to a biaxial compressive strain up to 1.0%.When calcium fluoride is employed as substrate 20, care must be taken tominimize exposure to water vapor at elevated temperatures of greaterthan about 600° C.

After forming the structure shown in FIG. 3E, various CMOS devices,including nFETS, pFETs and a combination thereof, can be formed directlyon the Si-containing layer 16. The CMOS devices are formed utilizingconventional processes that are well known to those skilled in the art.

In addition to the wafer transfer technique described above in FIGS.3A–3E, the present invention also contemplates an embodiment for forminga semiconductor material having a <110> Si-containing layer that isunder biaxial compressive strain wherein at least one multiply connectedtrench isolation region, a compressive liner or both are used to createthe strain in the Si-containing layer.

FIGS. 4A–4C show the embodiment in which both the at least one multiplyconnected trench isolation region and a compressive liner are used tocreate the strain in a Si-containing layer. The compressive liner isformed after isolation trench formation and formation of the CMOSdevices on the surface of the Si-containing layer or substrate 10.

This embodiment of the present invention begins by first providing aSi-containing substrate or layer 10 having a <110> crystal orientationand then forming at least one multiply connected trench isolation region50 in layer 10. Herein after the at least one multiply connected trenchisolation region is referred to as just isolation trench region. Theterm “multiply connected” means that the isolation regions have holestherein. The isolation trench regions 50 are formed by first forming ahardmask (not shown) on the surface of the substrate 10. The hardmasktypically comprises a nitride layer on top of a thin oxide layer. Thehardmask can be formed by a thermal growth process or deposition, bothof which are well known to those skilled in the art. The thickness ofthe hardmask layer can vary depending on the material and technique usedin forming the same. Typically, the hardmask has a thickness from about30 to about 100 nm.

Following formation of the hardmask, a patterned photoresist (not shown)having at least one multiply connected trench is formed by depositionand lithography. The at least one trench pattern is then transferred tothe hardmask layer by a conventional etching process. Following thepattern transfer, the patterned photoresist is typically removed fromthe structure by a conventional stripping process and then the trenchpattern formed into the hardmask is transferred to the substrate 10 viaanother etching process. This etching step forms a trench into thesubstrate 10. Alternatively, a single etch sequence can be used topattern the hardmask and form the trench into the substrate. The depthof the trench, as measured from the upper surface of the substrate 10 tothe bottom of the trench, is typically from about 50 to about 500 nm.

Following pattern transfer to the substrate 10, an optional trench liner(not shown) is formed so as to line the walls of the trench andthereafter the trench is filled by a conventional deposition processwith a trench dielectric material including for example an oxide. Afterthe trench fill step, the trench dielectric above the trench istypically removed via a planarization process and then the hardmask isremoved.

A densification step is typically conducted prior to planarization andhardmask removal. Typically, this is a long (hour-long) anneal at hightemperatures (900°–1100° C.) in an N₂ ambient. This essentially drivesoff the hydrogen in the oxide material.

The structure that is formed after the above steps have been performedis shown in FIG. 4A. At this point of the present invention, at leastone CMOS device represented by reference numeral 52 can be formed on theexposed surface of the substrate 10 by utilizing a conventional CMOSprocess. See, for example, the structure shown in FIG. 4B.

Following CMOS device fabrication, a compressive liner 54 is formed onthe exposed surfaces of at least the substrate 10. The compressive lineris typically comprised of a nitride-containing material. Althoughnitride-containing materials are typically used, other insulatingmaterials that can induced biaxial stress to the Si-containing substrate10 can be used. The compressive liner 54 is formed by a utilizing adeposition process such as PECVD or RTCVD. The thickness of thecompressive liner 54 can vary depending on the conditions used informing the same. Typically, the compressive liner 54 has thickness fromabout 20 to about 100 nm. The compressive liner 54 formed at this pointof the present invention introduces compressive stress into the regionunder the gate of the device. (see Region 55).

Following formation of the compressive liner 54, oxide layer 56 isformed by a deposition process such as PECVD. The thickness of oxidelayer 56 can vary depending on the conditions used in forming the same.Typically, the oxide layer 56 has a thickness from about 200 to about1000 nm. This oxide layer is then planarized using CMP. FIG. 4C showsthe resultant structure that is formed after formation of thecompressive liner 54 and oxide layer 56.

In this embodiment of the present invention, the trench isolationregions 50 produce compressive stress longitudinally towards the channel(and also laterally for narrow devices). The compressive stress in thechannel will be higher for shorter source/drain overhang regions.Different types of nitride liners with different stress can modulate thelocal stress of the channel.

It is again emphasized that although FIGS. 4A–4C show the presence ofboth the at least one multiply connected trench isolation region and thecompressive liner to create the biaxial compressive strain in theSi-containing layer, the strain can also be created using only the atleast one multiply connected trench isolation region or the compressiveliner.

It has been determined by the present applicants that the current willhave slightly more degradation on a <100> orientated wafer than a <110>oriented wafer for NMOS devices, and more enhancement on the <100>oriented wafer than a <110> oriented wafer for a pMOS. The sensitivityof the current change is not high on the nMOS with different nitrideliner stress, but is higher on the pMOS.

When the device becomes narrower, the channel will receive compressivestress from the trench isolation region in the lateral direction. FIG. 5shows that both NMOS and pMOS drive current will be degraded. Devices on(100) wafers will have larger degradation.

When the narrow device has small S/D overhang region, the channel willreceive compressive stress in both lateral and longitudinal directions.FIGS. 6A–6B show the change of saturation current of narrow widthdevices. For devices on (100) surface, the NMOS current will be degradedby a large S/D overhang region and improved by a smaller S/D overhangregion. This threshold region from mobility degradation to improvementindicates the effect from uni-axial to biaxial stress effect. Devices on(110) wafers have higher sensitivity than those on (110) wafers, and theimprovement can be as high as 155%. This suggests that the longitudinalcompressive stress plus the lateral stress, or simply biaxialcompressive stress, can enhance the NMOS current. The nitride liner canalso modulate the biaxial stress effect and is more effective on thedevices built on the (110) wafer and narrow width devices (FIGS. 7A–7B).FIGS. 7A–7B show that NMOS has current improvement instead ofdegradation with small S/D overhang region when the width is narrowerthan 0.2 mm. Similarly, pMOS will have higher mobility change fornarrower width devices compared to long width devices. Both uni-axiallongitudinal and biaxial stress can improve pMOS performance.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A semiconductor material comprising a silicon-containing layer havinga <110> crystal orientation overlying a substrate having a coefficientof thermal expansion that is greater than the coefficient of thermalexpansion of silicon, said silicon-containing layer is under a biaxialcompressive strain and having electron and hole mobilities that are bothgreater than those in a conventional unstrained <100> Si substrate. 2.The semiconductor material of claim 1 wherein the biaxial compressivestrain is greater than about 0.2%.
 3. The semiconductor material ofclaim 2 wherein the biaxial compressive strain is greater than about0.5%.
 4. The semiconductor material of claim 1 further comprising anoxide layer located between said silicon-containing layer and saidsubstrate.
 5. The semiconductor material of claim 1 wherein thesubstrate is sapphire, germanium, or calcium fluoride.
 6. Thesemiconductor material of claim 1 wherein the Si-containing layer is acrystalline Si-containing layer comprising Si or SiGe.
 7. Thesemiconductor structure of claim 1 wherein said silicon-containing layeris located directly atop said substrate.